1. Field of the Invention
The present invention relates to helical scan tape storage devices and more specifically relates to a helical scan drum design having read heads optimally positioned for reading data in non-tracking tape subsystem. The present invention further relates to simulation methods for optimally designing the placement of read heads on such a drum design.
2. Related Patents
The present invention is related to co-pending U.S. patent application entitled "Variable Speed Recording Method and Apparatus for a Magnetic Tape Drive", invented by Beavers et al., U.S. Ser. No. 09/176,079, filed Oct. 20, 1998, and co-pending U.S. patent application entitled "Fine Granularity Rewrite Method and Apparatus for Data Storage Device", invented by Zaczek, U.S. Ser. No. 09/176,015, filed on Oct. 20, 1998, and co-pending U.S. patent application entitled "Multi-level Error Detection and Correction Technique for Data Storage Recording Device", invented by McAuliffe et al., U.S. Ser. No. of 09/176,014, filed on Oct. 20, 1998, all of which are commonly owned and all of which are hereby incorporated by reference.
3. Discussion of Related Art
Tape storage devices are often used in storage applications where high density storage is required and removability of the storage medium is desired. For example, such tape storage subsystems are often used for data archive in computing systems. User and system data stored on a computer system's disk storage subsystem is copied to a tape medium in a tape storage subsystem. The tape medium is removable from the tape storage subsystem and can then be securely stored (for example off site) as a secured archive copy of critical data stored on the computer system's disk storage subsystem.
As computer storage needs have risen, so have demands for high density tape storage subsystems. Early tape storage subsystems stored data in parallel tracks running linearly the length of the tape medium. These systems are often referred to as longitudinal tape subsystems. Both the linear bit density (the density of magnetic flux changes along a single linear track) as well as the track density (the number of tracks placed across the width of the tape medium) affected the total storage density of data on the tape medium.
As physical limits were encountered in design of such linear tape devices, helical scan tape subsystems evolved to further increase tape medium storage densities. This is a recording format in which a relatively slow moving tape is helically wrapped around a rapidly rotating drum with an embedded record head and read head. The tape is positioned at a slight angle to the equatorial plane of the drum. This results in a recording format in which recorded tracks run diagonally across the tape from one edge to the other. The record head rotates past the tape spanning a diagonal from one edge to the other. As the drum rotates, the record head records another diagonal track with more data. Recorded tracks are parallel to each other but are each at an angle to the to edge of the tape. This geometry of discrete sized tracks on the magnetic tape medium allows still higher densities of data to be stored on the tape as compared to older linear (longitudinal) tape subsystems.
It is common in helical scan devices to use at least two record heads typically adjacent one another on the circumference of the drum. This allows two parallel helical scan tracks to be recorded during each rotation of the drum. Typically the two heads are referred to as an "A" head and a "B" head, respectively. The tracks recorded by each head are correspondingly referred to as "A" tracks and "B" tracks. "A" tracks are recorded by the "A" head at a first azimuth angle (an offset angle relative to the perpendicular of the angle of the tape relative to the equatorial plane of the drum). The "B" tracks are recorded by the "B" head at a different azimuth angle (typically 20-40 degrees offset from the "A" azimuth angle.
Typical helical scan tape devices also have one or more read heads for reading back the data recorded on the tape medium. The read head receives the magnetic flux changes previously recorded on the tape. Analog and digital electronic circuits then reconstruct the data represented by the recorded flux changes. Where multiple recording heads are used having different azimuth angles, there are corresponding read heads with identical azimuth angles for reading corresponding tracks. In other words, read heads are of the "A" type and "B" type having identical azimuth angles to the corresponding recording heads. Often a single head may serve the dual purpose of a read head and a write head.
Mechanical tolerances for such helical scan devices are extremely critical to proper operation due the higher track and bit densities of the format. The "A" read head must be substantially aligned with the "A" track to successfully read the data. In like manner, the "B" read head must be substantially aligned with the "B" track to successfully read the recorded data. Mistracking is the phenomenon that occurs when the path followed by the read head of the recorder does not correspond to the location of the recorded track on the magnetic tape. Mistracking can occur in both longitudinal and helical scan recording systems. The read head must capture a substantial percentage of the track in order to produce a playback signal. If the head is too far off the track, recorded information will not be played back.
Most helical scan tape devices use complex (hence costly) tracking circuits to assure that the appropriate heads are aligned over the corresponding recorded data. Servo feedback control circuits constantly monitor and control speed of the drum and tape to assure alignment of the heads and the tape. Special servo control data is usually recorded on the tape medium to enable the servo feedback circuits to resynchronize the tracking if the tape is stopped or reverses direction.
It is common to read data immediately after writing the data as a check of the quality of the data writing operation. This process is often referred to as check after write (or CAW). When tracking features are used in a tape device, write operations use the tracking features for speed control of the tape and drum. Read operations in tracking devices use the tracking circuits to precisely position the read head over the written track.
As noted, tracking circuits add significant complexity and associated cost to helical scan tape devices. Some helical scan devices are non-tracking in that they use no such expensive tracking circuits to assure alignment of the heads with the track. Rather, presently known non-racking tape devices significantly slow the tape speed relative to the drum to permit multiple passes of the read head over the same track. Each pass is at a slightly different longitudinal position on the tape due to the tape movement but because of the slower speed overlaps a portion of the track read by the previous pass. This overlap of sequential passes is often referred to as overscan. To achieve sufficient overscan to assure proper reading of the track by at least one of the read heads, such non-tracking devices reduce the speed of the tape to half of the nominal speed (i.e., half the speed at which the tracks were recorded). This permits a first pass read to overlap a second pass read thereby helping to assure that one of the passes will substantially cover the track width. However, slowing the tape for read operations negatively impacts read operation performance of the tape device.
It is therefore a problem in the art to achieve accurate reading of helical scan tracks on a tape device without resorting to complex, costly tracking circuits and without unduly slowing the performance of the tape device for read operations. In other words, it is desirable to perform overscan read operations on non-tracking tape devices at full speed.